1. Field of the Invention
The field of this invention broadly relates to coding and decoding binary data. More particularly, the field of this invention includes communication systems wherein data is transmitted from point to point. The invention is useful for transmission of binary data in a converted form over any communication medium. Thus, this invention is applicable to transmission of data over twisted pair telephone lines, telephone links (including switching networks, microwave, etc.) leased lines, or over the air communication links such as, for example, high frequency radio or microwave systems.
2. Description of the Prior Art
Digital data transmission is a well-known art. Numerous techniques have been employed in order to convert digital data into coded forms which are believed to be optimum for the communication link being employed. As a typical example, digital data transmission over various communication links have been accomplished by phase modulation, amplitude modulation, or a combination of phase and amplitude modulation.
For purposes of simplicity in discussion, several system approaches will be discussed with strict reference to amplitude levels. It should be understood at the outset, however, that the principles discussed herein are equally applicable to phase and other modulation forms in either baseband or carrier systems.
Certain accepted principles have been adopted in known digital data encoding and decoding schemes. These accepted principles have been considered inviolable prior to this invention. Thus binary data to be transmitted over any communication link in the prior art has the number of bits integrally related by a whole integer such as 1, 2, 3, 4, etc., to the encoding period that is occupied by the binary data to be transmitted. For example, in a two level system, a non-return-to-zero change (NRZC) binary data string of ONES and ZEROES in a random sequence is shown in row A of prior art FIG. 1. In this NRZC format a binary ZERO occupies an entire encoding period E.P..sub.1 as a low level, whereas a binary ONE occupies an entire encoding period (such as E.P..sub.5) as a high level.
Because of the high data transmission speed required for today's communication systems, it is known in the art to group serial binary data into various multi-bit groups and transmit the groups of information during an appropriate encoding period. For example, row B of FIG. 1 shows the incoming binary data grouped in pairs of bits, referred to as dibits. There are four possibilities for such dibit pairs, namely 00, 01, 10, 11. In such a system, four levels are required in order to amplitude encode the four possibilities.
In row B of FIG. 1, the dibit pair 00 is arbitrarily assigned a zero level and is grouped as one signal level in encoding period E.P..sub.1. The dibit pair 01 is assigned a first level, the dibit pair 10 a second level and the dibit pair 11 a third level. The bandwidth required for dibit transmission of row B is one-half of the bandwidth required for the two level amplitude encoding of row A.
In all systems discussed herein, the number of encoding periods per second is known in the art as a baud rate for the system. For example, in the straightforward binary format of row A, one bit occupies a full encoding period. If the incoming data is at a speed of 4800 bits per second, row A depicts a 4800 baud rate system. Many individuals simply refer to such a system as a 4800 baud system.
In row B, on the other hand, two binary bits or dibits occupy each given encoding period as contrasted with the one bit per encoding period for row A. If it is assumed that the incoming data is at a speed of 9600 bits per second, the four level format of row A allows two bits to occupy each encoding period. Thus row B, (even though the data speed is doubled) is still referred to as a 4800 baud system, because there are 4800 encoding periods per second. In fact, all known prior art coding and encoding formats disclose what has heretofore been considered an inviolable rule that the number of bits to be transmitted in any encoding period is always related to that encoding period by a whole integer. For simplicity's sake, the relationship between the number of bits to their encoding period will be termed a bits-to-baud ratio. Applying this definition to the prior art systems of rows A and B means that the NRZC format of row A has a bits-to-baud ratio of 1, whereas the four level format of row B has a bits-to-baud ratio of 2.
Rows C and D depict two known prior art three level systems. These systems are subject to the same rule that the bits-to-baud ratio is a whole integer, namely 1. Row D is referred to as a duobinary format. In a duobinary format the encoding rule simply states that a binary ZERO always occupies a zero level. A binary ONE, on the other hand, always occupies a plus level or a minus level. Adjacent binary ONES hold the plus or minus level depending upon whether or not the number of preceding ZEROS were even or odd.
Duobinary encoding format has been claimed by some that it lowers the bandwidth of the energy spectrum. However, it still requires a bandwidth of 1/2T where T is the encoding period. This requirement on bandwidth is in accordance with the well-known Nyquist rule. It should be noted that the duobinary format has the same bit-to-baud ratio as the NRZC binary format.
In row C an alternative three level prior art system is shown. The rule of this prior art system is that a binary ZERO always occupies a zero level whereas binary ONES alternate from a plus one value to a minus one value. Again, in this format the bit-to-baud ratio is exactly the same as for the NRZC format of row A.
All of the prior art formats discussed above possess certain advantages and certain disadvantages, based on a figure of merit which may be assigned to each system format. In every instance wherein digital data is transmitted by a transmitter to a receiver over a communication link, people skilled in the art are concerned with many different factors. Of primary concern for any given format are the required bandwidth, the signal-to-noise ratio and the tolerance to peak-to-peak phase jitter. There are, of course, other considerations such as signal distortion due to amplitude, delay variation, and frequency translation which must be taken into consideration. Each of the prior art formats discussed above represents tradeoffs in these various noise problem areas, but my format truly presents a remarkable compromise in all of these primary factors by deviating from the preconceived concept that the bits-to-baud ratio must be a whole number. In my system format the bits-to-baud ratio is a mixed number and I achieve improved performance in baseband and in carrier systems as well. Baseband transmission is utilized in local distribution, wherein data is transmitted and received from point to point over hard-wire such as twisted copper pairs of a telephone line.
Repeaters in local distribution systems are placed very closely together in such hard-wire lines so that the prior art formats may be continually reshaped without loss of the signal levels which represent encoded binary data. Such close spacing is required because the prior art formats require a broad bandwidth in comparison with my invention's narrower bandwidth.
Since my coding format reduces the bandwidth requirements the close spacing of repeaters for prior art formats is no longer necessary.
Additionally, my system format is very suitable for telephone link transmission, (i.e., including switching networks, microwaves, etc.). It has a signal-to-noise ratio better than the signal-to-noise ratio shown for the four level systems of row B of FIG. 1. Its tolerance to peak-to-peak phase jitter (in a carrier system) is several times improved over the four level system. At the same time it offers extreme flexibility in that the direct current level that is placed on a telephone line may be zero which is highly desirable for simplicity in alternating current coupling the signal to a twisted pair telephone line for local distribution.
In some instances, of course, it is desirable to place a direct current signal on the line in an encoded format. In such an event it is a simple matter in my system format to provide this capability without expensive equipment modification. Furthermore, I have provided a system capability which will transmit data with at least a 50% increase in speed over the two and three level systems of the prior art while using the same bandwidth as these prior art systems because the bit-to-baud ratio I employ is a whole number and a fraction, as will be described in more detail hereinafter.